Tire Size Box

325/50R18 vs 325/55R18

325/55R18 compared with 325/50R18

+4.2% diameter, 0.0" width

325/55R18 is 32.07" tall vs 30.80". More than the 3% rule of thumb, so check carefully before switching.

30.8"18" rim6.4"12.8" wide
325/50R18 (solid) compared with 325/55R18 (dashed outline), drawn to scale

With 325/55R18 tires, when your speedometer says 60 mph you are really going

62.5 mph

325/55R18 is +1.28 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/50R18325/55R18Difference%
Overall diameter30.80"32.07"+1.28"+4.2%
Section width12.80"12.80"0.00"0.0%
Sidewall height6.40"7.04"+0.64"+10.0%
Circumference96.75"100.77"+4.02"+4.2%
Revolutions per mile654.9628.8−26.1−4.0%

Speedometer difference

If your vehicle came with 325/50R18 and you fit 325/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 325/50R18 or 325/55R18?
325/55R18 is taller by 1.28 inches (32.07" vs 30.80"). 325/50R18 is the same width.
Can I replace 325/50R18 with 325/55R18?
It is not recommended without checking carefully. The diameter difference is +4.2%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 325/55R18 change my speedometer?
When your speedometer reads 60 mph with 325/55R18 fitted (calibrated for 325/50R18), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 325/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.64 inches of ground clearance at the axle.

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